The H index metric can be used for many entities - authors, journals, departments, even countries! It only requires a collection of cited articles over time. The H index is a measure of productively and impact and gets higher with the passage of time. The H index indicates that an author has published "X" number of articles, which have been cited at lest "X" number of times. So an H index of 12 means that the author has published at least 12 articles which have each been cited at lest 12 times. It is designed to look at overall productivity rather than just a citation count, which can be influenced by 1 or 2 highly cited articles.
This image shows a graphic example of an H Index of 6, meaning that the author has 6 articles (x line) which have each been cited at least 6 times (y line.)
This image shows a chart of both the number of articles (on the left) and the number of times cited (on the right.) The H index is where they are closest to each other numerically.